In a vehicle in which an engine (internal combustion engine) is mounted, as a transmission that appropriately transmits the torque and rotational speed generated by the engine to drive wheels according to the vehicle running state, an automatic transmission is known that automatically optimally sets a gear ratio between the engine and the drive wheels.
As an automatic transmission mounted in a vehicle, there are, for example, a planetary gear transmission that sets gears using clutches, brakes, and a planetary gear apparatus, and a belt-driven stepless transmission (CVT: continuously variable transmission) that steplessly adjusts the gear ratio.
In a vehicle in which a planetary gear automatic transmission is mounted, a gearshift map that has gearshift lines (gear switching lines) for obtaining an optimal gear according to a vehicle speed and a throttle opening degree (or an accelerator opening degree) is stored in an ECU (electronic control unit) or the like, a target gear is calculated with reference to the gearshift map based on the vehicle speed and the throttle opening degree, and based on that target gear, a gear (gear position) is automatically set by engaging or releasing a clutch, a brake, a one-way clutch, and the like, which are frictionally engaging elements, in a predetermined state.
Furthermore, in a belt-driven stepless transmission, a belt is wrapped around a primary pulley (input-side pulley) provided with a pulley groove (V groove) and a secondary pulley (output-side pulley). At the same time that the groove width of the pulley groove of one pulley expands, the groove width of the pulley groove of the other pulley narrows, and, thus, the belt contact radius (effective diameter) of the belt relative to each pulley is continuously varied to steplessly set the gear ratio.
As a control apparatus that controls such an automatic transmission, there is a so-called by-wire shift-switching apparatus (see PTL 1, for example) that electrically detects the position of the shift range of the automatic transmission using a sensor, and by driving an actuator of an electric motor or the like for shift switching based on this detection signal to switch a manual valve of the automatic transmission, switches a shift position such as P (parking), R (reverse), N (neutral), and D (drive).
In such a by-wire shift-switching apparatus, it is not necessary for a shift lever and a shift range-switching mechanism to be mechanically linked as in an ordinary shift-switching apparatus, that is, a shift-switching apparatus in which a shift range of an automatic transmission is directly switched by a driver operating a shift lever. Accordingly, the layout when mounting these constituent elements in the vehicle is not restricted, and, thus, there are the advantages that it is possible to increase the degree of freedom in design, and also that installation in the vehicle can be easily performed.
In a vehicle in which a by-wire shift-switching apparatus is mounted, there is a technique described in PTL 2, as a failsafe control performed when a problem has occurred in shift switching.
With the technology described in PTL 2, in an automatic transmission that employs a shift by-wire control, a gearshift range position (target range position) that has been obtained through a switching operation for shift selection performed by a driver is compared with an actual range position of the automatic transmission, and when these are not the same, a judgment is made that a problem has occurred in the automatic transmission. When judged that a problem has occurred in the automatic transmission, by blocking a power transmission path from an output shaft of the engine via the automatic transmission to the drive wheels, vehicle movement in a shift range position different from the shift position selected by the driver is prevented.
Also, with the technology described in PTL 2, for example, the power transmission path is blocked by releasing a forward frictionally engaging element (clutch) or rearward frictionally engaging elements (clutch and brake) of the automatic transmission.